Microfluidic devices have been used in biochemical fields to perform high throughput screening assays. Microfluidic devices provide fluidic networks in which biochemical reactions, sample injections, and separation of reaction products may be achieved. In many conventional microfluidic devices, fluid flow and reagent mixing are achieved using electrokinetic transport phenomena (electroosmotic and electrophoretic). Electrokinetic transport is controlled by regulating the applied potentials at the terminus of each channel of the microfluidic device. Within the channel network, cross intersections and mixing tees are used for valving and dispensing fluids with high volumetric reproducibility. The mixing tee may be used to mix proportionately two fluid streams in ratio from 0 to 100% from either stream simply by varying the relative field strengths in the two channels.
Unfortunately, the use of active (or external) forces to induce flow is cost prohibitive and overly complex. Thus, other microfluidic devices have also been developed. For example, U.S. Pat. No. 6,416,642 to Alajoki, et al. describes a device that utilizes a wick (which may be pre-wetted, dry or wetted in position in contact with a microfluidic system) that acts by capillary action to draw material through channels or wells in which it is placed in fluidic contact. Alternatively, or additionally, a volume of liquid is optionally injected or withdrawn downstream of the material or region of interest, and the flow rate modulated by creating a pressure differential at the site of injection. However, problems still exist with such conventional microfluidic devices. For example, the flow rate through the device may sometimes be too fast to handle the data acquisition or necessary reaction time.
As such, a need still exists for an improved microfluidic assay device that is relatively inexpensive, easy to use, and that is capable of effectively and accurately determine the presence or concentration of an analyte within a fluid test sample.